Post-Eruption Structure of Arched Plasmas: Role of Force Imbalance and Decay Index

The solar atmosphere is copious with arched magnetic structures. When subjected to magnetic force imbalance, an arched magnetized plasma can produce oscillatory or eruptive structures such as jets or coronal mass ejections (CMEs). We conducted laboratory experiments on the Solar Plasma Device (SPD) at UCLA using hot-cathode sources, designed to mimic conditions in the solar atmosphere to study the nature of post-eruption structures under a broad range of parameters [1]. The laboratory arched plasma (β ≈ 10^-3, Lundquist number ≈10²–10⁵, plasma radius/ion-gyroradius ≈ 20, B ≈ 500 - 1000 G at foot points, repetition rate = 0.5 Hz) evolves in an ambient magnetic field with a variable decay index. The decay index measures how quickly the ambient magnetic field decreases with distance from the arched plasma and plays a crucial role in the growth rate of the torus instability [2]. The eruptive and oscillatory behavior of the plasma was recorded using 2D measurements of magnetic field, plasma density, and electron temperature, with images captured via a CCD camera. We identified regimes of force imbalance with low decay index that reliably produce eruptive jets. Conversely, regimes with a much higher decay index favor an expanding arch eruption. Initial results suggest that eruptive events are associated with the destabilization of oscillatory structures and broad spectra of fast waves.